Production of moulds and castings

ABSTRACT

PRODUCTION OF A CERAMIC SHELL MOULD FOR USE IN METAL CASTING WHICH COMPRISES FORMING ON A DISPOSABLE PATTERN A SHELL MOULD, REMOVING THE PATTERN, FIRING THE MOULD, CASTING METAL INTO SAID MOULD, SOLIDIFYING SAID METAL AND REMOVING SAID MOULD FROM SAID SOLIDIFIED METAL. THE SHELL MOULD IS CHARACTERIZED BY CONTAINING A PARTIUCLATE CERAMIC REFRACTORY MATERIAL, A BINDING AGENT AND AN INORGANIC MATERIAL WHICH LOSES CARBON DIOXIDE UPON FIRING TO FORM AN OXIDE.

United States Patent Inventor Appl. No.

Priority Gordon Halsey Oswestry, England June 11, 1968 June 28, 1971Monsanto Chemicals Limited Monsanto House, London, England June 28, 1967Great Britain PRODUCTION OF MOULDS AND CASTINGS l 1 Claims, No DrawingsU.S.Cl 164/131, 164/26, 164/35, 164/138 Int. Cl 822d 29/00, B22c 9/04Field of Search 164/25, 26,

[56] References Cited UNITED STATES PATENTS 3,311,956 4/1967 Townsend etal 249/61X 3,459,253 8/1969 Woolcott 164/36 FOREIGN PATENTS 600,0933/1948 Great Britain 164/26 1,023,829 3/1966 Great Britain 164/26Primary Examinerl. Spencer Overholser Assistant Examiner.lohn E. RoethelAttorneys- Richard W. Sternberg, Roger R. Jones and James J. MullenABSTRACT: Production of a ceramic shell mould for use in metal castingwhich comprises forming on a disposable pattern a shell mould, removingthe pattern, firing the mould, casting metal into said mould,solidifying said metal and removing said mould from said solidifiedmetal. The shell mould is characterized by containing a particulateceramic refractory material, a binding agent and an inorganic materialwhich loses carbon dioxide upon firing to form an oxide.

PRODUCTION OF MOULDS AND CASTINGS This invention relates to theproduction of moulds, in particular to a process for the production ofceramic moulds for use in metal casting, and to a process of metalcasting employing ceramic moulds.

A problem that has in certain instances hindered the utilization ofceramic shell moulds for the manufacture of metal castings stems fromthe fact that the material of the mould can adhere tightly to thesurface of the casting. The problem is particularly acute where acasting is of such a shape as to give rise to trapped pockets of ceramicfirmly held by contraction of the metal on cooling, or where the metal(such as aluminum) is comparatively soft so that efforts to remove theceramic by hammering or shot-blasting can damage its surface. Thepresent invention is directed to a solution of the problem by providingmeans whereby the shell is rendered more susceptible to attack by wateror by aqueous acid or alkali so as to facilitate its removal from thecasting.

The invention comprises a process for the production of a ceramic shellmould for use in metal casting by forming on a disposable pattern ashell mould comprising a particulate ceramic refractory material and abinding agent, removing the pattern and firing the mould, in which thereis included in the fabric of the unfired mould a particulate solidsubstance that is substantially unattacked by the binding agent duringformation of the mould but is converted on firing to a material thatrenders the finished mould more susceptible to attack by water or byaqueous acid or alkali.

The invention also comprises a fired ceramic shell mould comprising abinding agent, a particulate ceramic refractory material and a materialthat renders the mould more susceptible to attack by water or by aqueousacid or alkali.

Also part of the invention is a process of casting a metal into aceramic shell mould according to the invention, removal of ceramic fromthe casting being assisted by subjecting it to the action of water or adilute acid or alkali.

The substance convertible on firing to a susceptible material issubstantially unattacked by the other slurry components, particularlythe binding agent. Normally it is substantially insoluble in water,firstly because a water-soluble substance would be prematurely attacked,and secondly because its dissolution in the binding agent could causethe latter to be unstable. Preferably it is crystalline ormicrocrystalline rather than amorphous.

A preferred class of inorganic materials or substances convertible onfiring to a susceptible material comprises metal carbonates that aresubstantially water-insoluble and which at the firing temperature losecarbon dioxide to form an oxide. The carbonates of calcium and magnesiumare preferred; strontium carbonate, for example strontianite, can alsobe used if desired although it would normally be excluded on account ofits relatively high cost. Dolomite, magnesite marble and calcite areoften convenient substances to use.

The susceptible material formed on firing need not be itself soluble inwater or in aqueous acid or alkali. For example, calcium oxide formedfrom calcium carbonate reacts with water to form slaked lime, thereaction being accompanied by disruption of the bond between the bindingagent and the oxide and by partial dissolution of the slaked limeproduced. This disruption effect can be used for example to increase theefficiency with which an alkaline solution can be used to attack asilica binding agent, or a dilute acid can be used to hasten attack onthe slaked lime.

The practice in the production of conventional ceramic shell moulds isto build up a shell of sufficient thickness by applying to the pattern.a succession of coats of particulate refractory material. The firstcoat is applied as a slurry to the refractory material in a liquidbinding agent, usually a silica sol or a hydrolyzed silicate estersolution, which is then gelled or set. This is followed by a coating ofparticles of a refractory material applied as a stucco to the gelledfirst coat while the latter is still damp. Slurry coatings and stuccoare then applied alternately until a shell of the required thickness hasbeen built up. The thickness of each slurry coating is determinedlargely by the viscosity of the slurry, thickness of from 0.2 mm. to 1mm. being usual. The assembly is then dried, the pattern is removed, forexample by melting or dissolving out, and the resulting shell is fired,usually at a temperature in the range of 950 C. to 1 C., although awider range, for example from 800 C. to l300 C. can be employed. Firingtimes normally vary from about 15-20 minutes to about 2 hours, dependingon the size and thickness of the mould. A period of about 1 hour istypical.

Each slurry coating and its associated stucco is commonly referred to asan investment layer, the first such layer being termed theprimary-investment layer, and the second and subsequent layers,secondary-investment layers.

in the usual manner of operating the process of the present invention,the particulate solid convertible substance is admixed with or replacesthe refractory material in one or more of the coatings applied to thepattern'in building up the shell. Most conveniently, the convertiblesubstance is used as or is included in a stucco coating, which procedurehas the advantage of reducing the probability of for instance acarbonate affecting the stability of an acidic binding agent. Apreferred feature is that the convertible substance be present as thestucco in the secondary-investment layers but not in theprimaryinvestment layer. The convertible substance is preferablydistributed substantially uniformly throughout, or forms, the stucco orslurry coating in one or more such layers.

The refractory materials generally available may be used, for example,Molochite, silica, zircon and alumina. For use in a slurry coating, itis usual for the refractory to be in a state of subdivision such thatnone is retained on a 200 British Standard Sieve (8.5.5.) mesh andpreferable that it should contain a substantial proportion of particlesthat are fine enough to pass through a 300 mesh British Standard Sieve(8.8.8.).

For use as a stucco coating, a somewhat coarser refractory material ispreferable; this, for example, can usefully contain more than 50 percentof particles having sizes in the range 40 to I00 B.S.S. mesh, althoughfiner material may also be present. The convertible substance used inthe process of the present invention should preferably have a particlesize range comparable with that of the refractory with which it is mixedor which it replaces.

The convertible substance can entirely replace the refractory materialin either the slurry orthe stucco used in forming one or more investmentlayers; preferably however it is mixed with refractory. Where theconvertible substance is used in admixture with a refractory materialwhether in the form of a slurry or a stucco, its proportion shouldpreferably exceed 10 percent by weight of the mixture, and morepreferably forms at least 25 percent, for example about 50 percent, byweight of the mixture.

The binding agent used in the production of a shell mould in the processof the invention can conveniently be a silicate ester such as forinstance ethyl silicate, for example the condensed ethyl silicate soldas Silester OS, or a silica sol having a concentration of between 20 and40 percent by weight of silica, such as for example those sold as Syton2X" and "Syton P (Silester and Syton are Registered Trademarks).

While the process herein described can be used for the production ofcastings of steels and other hard metals, its particular advantage liesin the fact that it permits satisfactory casting of comparatively softmetals, for instance on aluminum or magnesium alloy, from which theremoval of ceramic shell moulds has hitherto been difficult. Backingwith plaster or a granular refractory may be desirable where steel iscast.

The invention is illustrated by the following Example.

EXAMPLE Four wax assemblies were prepared in the form of l-inch squaresprues 10 inches long and with miscellaneous wax patterns attached.

One wax assembly was used to build up a ceramic shell mould in theconventional way using a primary coating of zircon flour bonded withcolloidal silica sol and five secondary coatings of Molochite No. 6bonded with hydrolyzed ethyl silicate. Molochite No. 10 material wasused as a stucco on all slurry coatings.

The other three wax assemblies were used to make ceramic shell mouldsthe only difference in technique being that crushed marble, theparticles of which varied in size from 10 to 30 mesh was mixed with theMolochite stucco in the secondary-investment layers. The stucco used informing the secondary layers of the second mould contained 10 percent byweight of marble while for the third and fourth moulds this proportionwas 50 percent by weight.

After four layers had been applied to the assemblies, the latter werejoined together in pairs by wax patterns of a pouring cup, and twofurther coatings of slurry and stucco were applied. The assemblies weredried, subjected to a temperature of 150 C. to melt out the wax, andthen fired at a temperature of lOC. for 1 hour.

Using the first two moulds thus produced, castings were made from a 0.4percent carbon steel, and the cold moulds were immersed in water forminutes. It was found that the first mould was unaffected by the watertreatment whereas the second mould was considerably softened and couldmore easily be removed'by shot-blasting.

The third mould was backed with a granular refractory and used toproduce a 0.4 percent carbon steel casting. The mould disintegratedalmost completely on immersion in water for 5 minutes and could bewashed off the casting by ajet of water.

The fourth mould was used to make a casting in an l l percentsilicon-aluminum alloy, the backing with granular refractory being foundunnecessary for casting this metal. Again it was found that 5 minutesimmersion in water caused almost complete disintegration of the mould,permitting removal without damaging the surface ofthe casting in anyway.

These results demonstrate the improvements obtained by incorporatingmarble in a shell mould in accordance with the invention.

It is to be understood that it is also within one scope of the presentinvention that the processes described herein in casting various metalsincluding casting metals such as highchromium steels, plain low-carbonsteels, for example, BS 1617A and CLA 9, low-alloy steels, for example,those of the Fortiweld variety and type EN 36C, high-carbon high-alloytool steels generally containing around 12 percent chromium and otheralloys which are commonly utilized in the art of casting.

It is also to be understood that it is within the scope of the presentinvention that the particular inorganic material such as calciumcarbonate is admixed with or replaces the refractory material in one ormore of the coatings applied to the pattern in building up the shell.Most conveniently, the inorganic material is used as or is included in astucco coating. A preferred feature is that the inorganic material bepresent as the stucco in the second of the secondary investment layersor in one or more subsequent layers but not in the primary investment orthe first of the secondary-investment layers.

I claim:

1. A process for the production ofa ceramic shell mould for use in metalcasting which comprises the steps of (l forming on a disposable patterna shell mould comprising a particulate ceramic refractory material, abinding agent selected from the group consisting of colloidal silicasols and alkyl silicates, and a particulate solid substance that issubstantially water insoluble and substantially unattacked by thebinding agent during formation of the mould, and loses carbon dioxideduring the firing step to form an oxide, (2) removing the pattern and(3) firing the mould whereby a mould is obtained which, when subjectedto a treatment by an aqueous medium, may readily be removed from themetal casting.

2. A process according to claim 1, in which the particulate solidconvertible substance is a metal carbonate that is sub stantiallywater-insoluble and which at the firing temperature loses carbon dioxideto form an oxide.

3. A process according to claim 2, In WhlCh the particulate solidconvertible substance is selected from the group consisting of calciumcarbonate and magnesium carbonate.

4. A process according to claim 3, in which the particulate solidconvertible substance is used as or is included in one or more stuccocoatings applied to the pattern in building up the shell.

5. A process according to claim 4, in which the particulate solidconvertible substance is included in one or more of the secondaryinvestment layers but not in the primary-investment layer ofthe ceramicshell mould.

6. A process according to claim 5, in which the particulate solidconvertible substance is used as an admixture with the particulaterefractory material, said admixture containing at least 25 percent byweight of the particulate solid convertible substance.

7. A process for the production of a metal casting which comprises thesteps of (l forming a shell mould on a disposable pattern by applyingalternative coatings of (a) a slurry ofa particulate ceramic refractorymaterial and a binding agent selected from the group consisting ofcolloidal silica sols and alkyl silicates, and (b) a stucco of aparticulate ceramic refractory material per se thereto, said shell mouldcontaining, in addition to items (a) and (b), a particulate solidsubstance that is substantially water insoluble and substantiallyunattached by the binding agent which losses carbon dioxide upon firingand forms an oxide and which is admixed with or replaces such refractorymaterial in one or more of said coatings applied to said pattern inbuilding up said shell to a desired thickness; (2) separating saiddisposable pattern from said shell mould; (3) heating said shell mouldfor a sufficient period of time and at a temperature in order tosubstantially harden said shell mould; (4) casting metal into thehardened shell mould; (5) solidifying said metal; and (6) removing saidmould from said solidified metal, said removal being effected at leastin part by subjecting said metal-containing mould to the action of anaqueous media.

8. The process as set forth in claim 7, wherein one coating of item (a)plus one coating of item (b) constitutes a layer, and the particulatefusible material constitutes or is part of the stucco coating of one ormore of the layers applied to said disposable pattern starting with thethird layer.

9. The process as set forth in claim 8, wherein the stucco coating ofone or more of such layers is a mixture of said solid substance and saidrefractory material, said solid substance being greater than 10 percentby weight of the total weight of said mixture.

10. The process as set forth in claim 9, wherein the solid substance isa metal carbonate which constitutes from about 25 percent to about 50percent by weight ofsaid mixture 11. A process as set forth in claim 10,wherein the metal that is cast is selected from the group consisting ofaplain lowcarbon steel, a low-alloy steel, a high-carbon high-alloy toolsteel, a ferritic stainless steel and a martensitic stainless steel.

ma Um-Ensnms PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 3, 587{/13 Bag June 28 197 l Inventofla) Gordon Halsey It is certified thaterror appears in the above-identified patent and thlt 1d Letters Patentno hereby contracted as shown below:

In column L, claim 7, line 55, the word "unattached" shouldread---unattacked-.

Signed and sealed this 12th day of September 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER,JR.

Commissioner of Patents Attesting Officer

